The invention relates to gas springs.
Gas springs embodying the invention and to be described below by way of example can be used in motor vehicles for easing the opening of closure members such as luggage and engine compartment lids and hatchbacks. However, they can be used for other purposes.
A known form of gas spring comprises a gas compression spring for moving a member pivotted on a body from a first position corresponding to compression of the spring to a second position, comprising: an arrangement of a piston movable within cylinder means having first and second ends, the piston carrying a piston rod slidingly and sealingly extending outwardly of the first end of the cylinder means; means on the second end of the cylinder means and at a point on the piston rod where it extends from the cylinder means for connection between the body and the said member; the cylinder means being filled with gas under pressure and the piston dividing the interior of the cylinder means into a first chamber defined at least in part by the head of the piston and a second chamber through which the piston rod extends; the first and second chambers being interconnected by first gas flow means providing a first restricted gas flow path between points which are respectively situated in the first and second chambers and which are separated by respective predetermined parts, only, of the stroke of the piston, the restricted gas flow path permitting limited flow of gas from the second chamber to the first chamber as the gas pressure moves the piston over the said predetermined parts of the stroke and correspondingly moves the piston rod from an inner piston rod position towards an outer piston rod position more outwardly of the cylinder means, whereby to move the member towards the second position, the said limited flow of gas not being permitted when the piston is outside the said predetermined parts of its stroke so that further movement of the piston rod towards the outer piston rod position is stopped; and reverse flow means permitting gas pressure transfer between the first and second chambers when the piston rod is moved towards the inner piston rod position by an externally applied force.
Such a gas spring is known from United Kingdom patent specification No. 1,528,820. In this gas spring, the first gas flow means comprises two grooves in the interior wall of the cylinder means which extends longitudinally along the cylinder means and are separated by an ungrooved portion. As the piston rod travels towards the outer piston rod position, the piston travels over the first groove permitting limited flow of gas from the second chamber to the first chamber. When the piston reaches the ungrooved portion, piston rod movement stops. The piston rod can only be moved beyond this position, so that the piston then reaches the second groove which again permits restricted gas transfer, by the application of an externally applied force which has to be sufficient to overcome the gas pressure within the cylinder means. Th e invention aims to deal with this problem.